Modification of high molecular weight substances has been performed by various chemical reactions to change the properties of the high molecular weight substances and to impart new functions to the high molecular weight substances. One example of such a modification is cyanoethylation with acrylonitrile. The most representative product obtained by such a cyanoethylation is a cyanoethylated cellulose. Due to its good solubility in many solvents, high dielectric constant and transparency, the cyanoethylated cellulose is used as a binder for illuminants of dispersion-type electroluminescent elements. The cyanoethylated cellulose is also expected to use as a condenser film. The cyanoethylated cellulose, however, has poor adhesive properties to various high molecular weight substances, metals and glasses. Therefore, where the cyanoethylated cellulose is used as a dielectric material or as a binder thereof, the cyanoethylated cellulose does not closely bond to electrodes thereby causing air spaces therebetween. As a result, there is disadvantage such that the high dielectric constant of this material is not utilized effectively.